This invention relates to a method of preparing single crystals of ferroelectric strontium barium niobate (hereinafter abbreviated as SBN).
As is well known in the art, SBN is a ferro-electric substance having a tungsten bronze structure, that is an opto-electronic substance characterized by an extremely large linear electro-optic coefficient, an extremely smaller half wave voltage than any known material, a maximum pyroelectric coefficient among known substances, and extremely large nonlinear polarizability for photoelectric constant and second harmonic generation. Owing to these excellent characteristics application of this material to make various optical devices and elements has been excepted greatly but practical use has not been succeeded because it has been impossible to obtain single crystals of this substance having excellent optical quality.
Single crystals of SBN prepared by prior art methods have optical defects such as cores and growth striae mainly caused by the variation in the composition. These defects greatly impair the homogeneity of the crystals, thus decreasing the extinction ratio. These defects are caused mainly by the difficulty of preparing homogenous solid solution crystals.
SBN is a solid solution consisting of three components of strontium oxide (Sr 0), barium oxide (Ba 0) and niobium pentoxide (Nb.sub.2 0.sub.5) and has a fairly wide solid solution area. As will be described hereinafter, when preparing solid solution crystals from a melt, it is essential to select a melt having a composition that is close to a congruently melting composition in order to always obtain single crystals having excellent optical characteristics.
The congruently melting composition has already been reported in publications, for example, J. R. Carruthess and M. Grasso "Phase Equilibria Relations in the Ternary System Ba0-Sr0-Nb.sub.2 0.sub.5 ", J. Electrochem., Vol. 117 (1970), pp 1426- 1430. Single crystals prepared in accordance with the values reported in this reference not only have the optical defects described above but also difficult to always prepare products of desired characteristics. This means that the reported value does not correspond to a true congruently melting composition so that it is essential to determine the true value in order to prepare crystals of excellent quality.